The present invention relates to a method of producing a molecular composite material comprising a rigid aromatic polythiazole and a matrix polymer, which molecular composite material is suitable as a structural material for aircrafts, automobiles, spacecrafts, etc.
Recently, to achieve the reduction of weight of aircrafts, automobiles, etc., plastic materials called "engineering plastics" having excellent mechanical properties, heat resistance, etc. have been increasingly used. Apart from this, development has been actively carried out to provide composite materials such as FRPs consisting of plastic materials and high-strength, high-modulus fibers such as carbon fibers, and these composite materials are being used in many applications.
It is known that the strength of these composite materials largely depend not only on the strength of plastics and reinforcing fibers but also on the interfacial strength of the fibers to the matrix resins. Also, the impregnating capability of matrix resins into the fiber-reinforced preforms affect the ease of production of composite materials and the strength of the resulting products. Accordingly, even though high-strength, high-modulus fibers and resins are used as starting materials, composite materials having excellent strength are not necessarily obtained.
Under such circumstances, to overcome the above problems, proposals have been made to develop high-strength polymer blend composites (molecular composite materials) by finely dispersing rigid polymers such as aromatic polyamides, etc. in matrix resins to a molecular level.
Aromatic polymers suitable for molecular composite materials include those containing heterocyclic repeating units such as thiazole rings, imidazole rings, oxazole rings, etc. Among them, aromatic polythiazole having a thiazole ring is highly promising because of its excellent mechanical strength.
In the meantime, the homogeneous dispersion of a reinforcing polymer in a matrix polymer cannot be achieved if the reinforcing polymer is simply blended with the matrix polymer. This means that simple blending fails to provide a molecular composite material having excellent mechanical properties. Therefore, various attempts have been made so far.
For instance, Japanese Patent Laid-Open No. 1-287167 discloses a method of producing a polymer composite comprising the steps of introducing a polymer solution mainly containing (A) a reinforcing polymer consisting of polythiazole having a substantially rod-shaped skeleton and (B) a fusable matrix polymer into a solidification bath, and forming it into a film, the above polymer solution showing an optical anisotropy and being solidified via an apparent, optically isotropic phase after immersion in the solidification bath.
Japanese Patent Publication No. 2-7976 discloses a polymer composition comprising a reinforcing polymer A consisting of polythiazole having a substantially rod-shaped skeleton, and a matrix polymer B consisting of a less-crystallizable aromatic copolyamide having a glass transition temperature of 200.degree. C. or higher and a flow initiation temperature of 500.degree. C. or lower, a weight ratio A/(A+B) being 0.15-0.70. When the aromatic copolyamide is kept at a temperature between its glass transition temperature and its flow-initiation temperature for an arbitrary period of time within 5 hours, the resulting crystals have apparent sizes of 25 .ANG. or less.
However, in the methods of producing polymer composites as disclosed in Japanese Patent Laid-Open No. 1-287167 and Japanese Patent Publication No. 2-7976, homogeneous dispersions of the reinforcing polymers in matrix polymers are not expected. This means that the resulting molecular composite materials do not show largely improved mechanical strength, etc. This appears to be due to the fact that the rigid reinforcing polymers and the matrix polymers do not show good compatibility with each other.
Thus, proposals have been made to provide a method of producing a molecular composite material of a rigid aromatic polymer without resorting to mixing the rigid aromatic polymer and a matrix polymer in an acidic solvent, the method comprising the steps of homogeneously mixing a prepolymer of the rigid aromatic polymer and a matrix polymer or its prepolymer in an organic solvent, removing the organic solvent and then heating it so that the rigid aromatic polymer is formed from its prepolymer (Japanese Patent Laid-Open Nos. 64-1760 and 64-1761). By the above methods, molecular composite materials having relatively good mechanical strength, etc. can be produced.
However, since these molecular composite materials fail to show sufficiently high mechanical strength, further improvement is desired. Insufficient mechanical strength of the conventional molecular composite materials obtained by using the aromatic polythiazole prepolymers is due to the fact that in the process of producing the aromatic polythiazole prepolymers, part of thiol groups in the prepolymers is subjected to a thiazole ring-closure reaction or a cross-linking reaction, forming insoluble components, and that therefore good dispersion of aromatic polythiazole cannot be achieved.